A day before the final World Cup match, 28 people  opposed to hosting the tournament in Brazil were arrested “preemptively” at their homes in the city of Rio de Janeiro on the early morning of July 12. Police suspected  they would engage in violent acts during a protest scheduled for the next day and accused them of “forming an armed gang” based on  what activists  and alternative media  are calling false evidence .
A total of 37 people were arrested  as part of Operation Firewall; some were detained simply for having a connection to the activists. Most were released, but five are still in jail waiting to be brought before the court or indicted.
Police reportedly found weapons, masks and explosives at some of the homes of those arrested, but activists have disputed the claim, saying that only knee pads, a tear gas mask, newspapers and a flag were seized. A 16-year-old, one of two minors detained, was accused of forming an armed gang based on a gun belonging to her father discovered in the house she was in, according to the collective Rio na Rua .
Those jailed before the World Cup final match were also accused of belonging to or helping the so-called Black Blocs , who have attended some demonstrations since 2013. “Black Bloc” describes the tactic of wearing black clothing and covering the face with a mask or bandanna, and some protesters dressed this way have vandalized property or committed arson.
The activists have denied being part of any Black Bloc group or engaging in any violent activity.
This isn't the first time during the World Cup that police have accused protesters of being Black Blocs. Fabio Hideki Harano  was arrested on June 26 after a demonstration against the mega-event in São Paulo. Police said he was carrying an explosive , but haven't produced evidence of it. He remains in jail.
Amnesty International and the Order of Attorneys of Brazil have criticised  the latest arrests, calling them illegal and a method of intimidation. One activist and lawyer , Eloisa Samy Santiago, who was arrested  asked unsuccessfully for asylum at the Consulate of Uruguay in Rio de Janeiro after she was released.
It is believed that these activists were targeted by authorities because they have taken part in past street demonstrations since June 2013 , when massive anti-government protests rocked the country. Their demands vary, as well as their political affiliation. Some are members of socialist or anarchist groups, and some have no affiliation whatsoever.
In 2013, protesters demanded free or reduced fare for bus tickets, rallied against the treatment  of those living in the slums, and called for the impeachment of Rio de Janeiro's mayor and governor of Rio de Janeiro. This year, activists railed against the public money spent on the World Cup and the forced evictions  for infrastructure construction that left some people homeless.
Eduardo Viveiros de Castro, one of the most famous anthropologists in the country, detailed  the police operation quoting a friend who preferred to stay anonymous:
A policia esta com mandados de prisão indo na casa de educadores, manifestantes com alegações mentirosas e variadas (sequestro, vandalismo, crime de internet).. estão t[am]b[em] seguindo pessoas na rua através de grampo no celular e prendendo… não se sabe o numero de militantes, mas não para de chegar gente na cidade da policia…
The police are going to the homes of educators and protesters with arrest warrants with false and varied allegations (kidnapping, vandalism, Internet crime) and are also following people in the street through bugs on their mobile [devices] and arresting… no one knows the number of activists, but more and more are arriving at Police City…
According  to Pablo Ortellado, a professor and leading figure within many social movements in Brazil, teachers and free media and anti-Cup activists were among those taken in. Alternative newspaper The New Democracy reported  that relatives and friends of those arrested were “threatened and intimidated at the police station.”
The arrests appeared to be an effort to prevent protests during the final day of the World Cup at Maracanã stadium. Targets were usually leaders or prominent figures of previous demonstrations who participated in last year's mass protests . Global Voices reported  in June on similar preemptive arrests of activists even the games began.
O objetivo imediato dessa ação, tão ilegal e arbitrária como várias outras que acontecem no país desde os protestos que começaram em junho do ano passado, parece ser intimidar e amedrontar as pessoas dispostas a participar dos protestos que estão sendo convocados para amanhã, durante a partida final da Copa do Mundo, no Maracanã. Para isso, está sendo utilizada a legislação de exceção aprovada aos níveis federal e estadual desde o ano passado.
The immediate objective of this action, as illegal and arbitrary as several others that have happened in the country since the protests began in June last year, seems to be to intimidate and scare people willing to participate in the protests tomorrow during the final match of the World Cup at Maracanã. For this, an legal exception approved at the federal and state levels since last year is being used.
Luiz Eduardo Soares, a leading expert on public security in Brazil, agreed :
A manobra visa evitar manifestações na final da Copa. Arrogância, incompetência e corrupção degradaram nosso futebol e estão arruinando as instituições que se pretendiam democráticas. É hora de denunciar e resistir.
The move aims to prevent demonstrations during the Cup final. Arrogance, incompetence and corruption eroded our football and are ruining the institutions that pretended to be democratic. It's time to denounce and resist.
A local judge in the 27th Criminal Court of Rio, Flavio Itabaiana de Oliveira Nicolau, gave the order to make the preemptive arrests. Filmmaker Fernando Marés tweeted a copy of the judge’s decision:
Decisão do juiz da 27ª Vara Criminal do Rio: prisões tentam impedir protestos marcados pra amanhã. pic.twitter.com/PlSAC7b9Ro 
— Roteiro de Cinema (@roteirodecinema) 12 julho 2014 
Decision of the judge of the 27th Criminal Court of Rio: Arrests try to prevent protests scheduled for tomorrow.
The Jornal Zona de Conflito Mídia Independente  posted a list  of activists who were arrested and those who were considered fugitives. Luis Rendeiro, considered a fugitive, sent a message on Facebook  to the newspaper The New Democracy saying police have accused him of a plot to bomb Maracana stadium. “They are harassing me and making bizarre accusations” in front of his wife and 3-year-old son, he wrote.
On Facebook, activist Renata Gomes was  direct in her description of what was happening: “Temporary detention warrants are the SCANDAL OF SCANDALS! To prevent people from demonstrating today! Someone please watch over the body of Brazilian democracy, because she's dead!”
Another activist, Caio Almendra, summarized  the situation:
Ativistas suspeitos de serem capazes de um dia, em algum momento, por alguma razão ainda não estabelecida, cometer atos violentos são detidos sem acusação clara, direito a advogado, com flagrantes forjados.
Viva o estado de direito!
Activists suspected of being able to one day, at some point, for some reason not yet established, commit violent acts are held without clear accusations, access to lawyers, with false evidence.
Long live the rule of law!